Overview Of The Phylum Arthropoda

The Arthropoda is a phylum more diverse than any other living or extinct animal taxon. Counted among this immense assemblage are beetles, butterflies, silverfish, centipedes, scorpions, mites, sea spiders, crabs, sow bugs, and barnacles, and many other common names too numerous to mention. Arthropods are the numerically dominant metazoan on land and rank among the most prominent benthic (bottom-dwelling)

and planktonic members of freshwater and marine ecosystems. They colonize virtually every conceivable habitat—from the equator to the poles, from high mountains to deep ocean trenches, and from rain forests to deserts and hot springs—and fill all trophic niches above the level of primary producer. Parasitism, especially ectoparasitism, is common in some groups, but most species are free-living. They range in size from tiny gall mites (80 |lm) to Japanese spider crabs with leg spans of 3.6 m. While some arthropods are vectors for human disease-causing organisms and others are major agricultural competitors with humans, they are also vital to the functioning of most ecosystems and a boon to humans in many ways. In addition to deriving nutrition from some arthropods (e.g., directly or indirectly from bees, crabs, lobsters, and shrimp), humans probably could not survive without arthropods.

The name "Arthropoda" is from the Greek, meaning "jointed foot." The presence of jointed appendages is the primary feature distinguishing arthropods from other phyla. Advantages provided by these appendages, a metameric or segmented body, and a hard skeleton are the three most important reasons for the phylum's success. Arthropods are segmented like annelid worms, but the evolutionary trend has been to fuse several metameres into body regions (tagmata) with specialized functions. Spiders have two tagmata, insects have three, and many crustaceans have two; however myriapods (millipedes and centipedes) lack tagmata. Arthropods have chitinous and proteinaceous exoskele-tons that are frequently strengthened with calcium salts. A modest, nonchitinized endoskeleton of inwardly projecting apodemes aids muscular attachment. To allow for continued somatic growth, the exoskeleton is shed periodically during ecdysis, a relatively strenuous and often dangerous process. Modifications of the exoskeleton have permitted arthropods to fly, swim, run, and burrow effectively.

Except for the molluscan cephalopods (e.g., the octopus), arthropods surpass all invertebrates in internal organ complexity. Although they are a phylum with a coelom, this structure no longer serves as a hydrostatic skeleton (as in annelids) but persists only as a cavity surrounding reproductive and/or excretory organs. The principal body cavity is instead the hemocoel, which is derived from the circulatory system. The open circulatory system consists of a dorsal heart, blood sinuses, and one or more discrete vessels. Hemocyanin and hemoglobin are the principal oxygen-carrying blood pigments. Respiration is achieved through the skin surface in some small species, but with gills in most aquatic organisms and tracheae and/or book lungs in terrestrial species. Excretory and osmoregulatory organs vary in type in accordance with the typical environmental moisture and salt content, as do the primary excretory products (ammonia in water and usually either uric acid or guanine on land). Cilia are absent externally and internally. The neural system is highly developed, with brain centers and complex sensory organs; indeed, next to vertebrates and cephalopods, the arthropod brain is the most complex on earth.

Most species reproduce sexually and are primarily dioecious (i.e., with an individual being a single gender), although parthenogenesis occurs in many taxa. Courtship and brood care are uncommon but are found in some members of all subphyla. Rather than possessing the spiral cleavage typical of many other protostomates, arthropods usually develop by superficial cleavage of a cytoplasmic layer above a yolky sphere. Larvae or discrete juvenile stages are common in terrestrial and aquatic taxa, but aquatic larvae never resemble the trochophore larvae that characterize related phyla.

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